The present invention relates to a presensitized plate requiring no dampening water for use in making a lithographic printing plate requiring no dampening water which permits printing without using any dampening water.
A lithographic printing plate which permits printing operations without using any dampening water (hereinafter referred to as "water-less lithographic printing plate") can be obtained by exposing a presensitized plate requiring no dampening water (hereinafter referred to as "water-less PS plate") to light and then developing the same with a developer. A variety of water-less PS plates have been proposed (see, for instance, Japanese Patent Publication for Opposition Purpose (hereunder referred to as "J.P. KOKOKU") Nos. Sho 44-23042, Sho 46-16044, Sho 51-17081, Sho 54-26923, and Sho 56-80046 and Sho 55-22781). Among them, a positive-working water-less PS plate in general has a light-sensitive layer comprising a photopolymerizable light-sensitive composition as described in J.P. KOKOKU Nos. Sho 54-26923 (U.S. Pat. No. 3,894,873) and Sho 56-23150.
Such a photopolymerizable light-sensitive layer in general has the following composition:
(1) an ethylenically unsaturated monomer or oligomer having a boiling point of not less than 100.degree. C.;
(2) a photopolymerization initiator;
(3) an optional heat polymerization inhibitor; and
(4) an optional polymer binder or inorganic powder used as a filler for maintaining the shape of the resulting light-sensitive layer.
The ethylenically unsaturated monomer or oligomer usually employed is a low molecular weight liquid or a solid having a low melting point and thus it is difficult to maintain predetermined hardness and thickness required for the light-sensitive layer, by simply using these ethylenically unsaturated monomers or oligomers.
For this reason, a polymer which has good compatibility with the ethylenically unsaturated monomer or oligomer is generally added to the light-sensitive composition as a polymer binder. Examples of such a polymer binder are vinyl polymers, (meth)acrylate polymers, unvulcanized rubbers, polyethers, polyamides, polyesters, polyurethanes, epoxy resins, urea resins and alkyd resins as disclosed in J.P. KOKOKU No. Sho 56-23150.
The inventors of this invention have conducted various studies of these polymers (hereunder referred to as "binder(s)") and have found out that a polyurethane of a polyol and a polyisocyanate is excellent as a binder and that the resulting water-less PS plate possessing the light-sensitive layer containing the binder exhibits in particular high resistance to scratch during plate-making processes and provides a water-less lithographic printing plate having high printing durability during printing processes.
However, if the foregoing polyurethane is used as a binder and the ethylenically unsaturated monomer or oligomer is employed in an amount greater than a predetermined level, the resulting light-sensitive layer becomes sticky and hence the problem of adhesion of the light-sensitive layer to rollers during plate-making processes arises. In addition, if the foregoing polyurethane is employed as a binder, various drawbacks arise. For instance, the adhesion of non-exposed portions of the light-sensitive layer to a substrate becomes insufficient, the light-sensitive layer of the non-exposed portions is peeled off during printing processes and, as a result, uneven image blinding occurs due to the difference in ink receptivities between the light-sensitive layer of the non-exposed areas and the substrate.
On the other hand, the plate-making process for water-less PS plates in general requires a step (step for checking the resulting plate) for dyeing the light-sensitive layer (so-called image portions) exposed through a development processing and for inspecting the dyed images in order to detect and/or confirm the reproduction of fine half-tone dots and the presence or absence of pinhole-like defects on the silicone rubber layer, but if this checking step is applied to the light-sensitive layer comprising the foregoing polyurethane as a binder, a sufficient density of the dye cannot be obtained.